Just as power transmission lines may convey transient surges or other voltage irregularities to electrical equipment intended to be powered therefrom, so equipment in line therewith may cause interference to be conducted back to an intervening electrical bus and to power lines themselves. Such conducted emissions (CE) may degrade computer performance, television picture quality, and the functioning of other electrical equipment powered from such lines. CE types of interference may be generated by computers, radio and TV receivers, motors, switching apparatus, and almost any electrical equipment with (or causing) a varying or noisy emission, whether as intended output or otherwise.
Filters are useful in reducing such interference, and it is conventional--in determining the desirability and effectiveness of filters--to measure conducted emissions with an instrument often called an EMI meter or spectrum analyzer. Such a meter has a logarithmic (dB) scale and is responsive via a voltage probe or current loop throughout a frequency range up to 30 MHz or higher.
Plotting the results of such measurements reveals whatever interference peaks and bands may exist, so that adequate filters can be designed and connected to (or installed in) the equipment under test (EUT) to limit such interference to a tolerable level at all frequencies. Designing such filters may fairly be viewed as principally an art rather than a science at this time.
Examples of persons who are both workers and writers in this field include Lon M. Schneider and Alphonse A. Toppetto, as well as the present inventor. Their published articles are of more interest than are EMI patents, so far as it known at this time. Noteworthy is the distinction they draw between "common mode" (CM) current conducted from phase and neutral lines to ground, and "differential mode" (DM) current conducted from a phase line to a neutral line. However, a need exists for methods and means of empirically determining with greater facility what filters are most suitable to combat both such types of conducted emissions. My invention is directed toward meeting that need.